My father squeezed fresh OJ every morning all his life. He only used juice oranges. They tend to be smallish, with mottled coloring and will not win a beauty contest - they seem very unpromising. But they yield the most juice and the most flavorful juice.

Valencia oranges. Thin skinned with lots of seeds make them difficult eating oranges but they have superior flavor and are excellent for juicing. I even augment prepared juice by juicing a couple of them into the amount I'm serving.